In recent years, R&D efforts have been intense on both LED chip and phosphors for phosphor-converted LED (pcLED), with the result that both efficient high-power LEDs and efficient phosphors have been demonstrated. However, a unique aspect of the phosphors operating in pcLED is that the phosphors are in close vicinity of the LED chip, and the LEDs operate at high temperatures. Typical junction temperatures of high power LEDs are in the range of 100° C.-150° C. At these temperatures, the crystal of the phosphor is at a high vibrationally excited state, causing the excitation energy to be directed to heat emission through lattice relaxation rather than to the desired luminescence emission. Moreover, these lattice relaxations produce heating with vibrational excitation, and thereby further reduce the luminescence emission efficiency. This is a vicious cycle that precludes successful applications of existing phosphor materials. The pcLED lamp for general illumination application requires high optical energy flux (e.g., higher than 1 Watt/mm2), which causes additional heating by a Stokes shift generated inside the phosphor crystals. Successful development of pcLED lamps for general illumination, therefore, requires phosphors that can operate highly efficiently at temperatures of 100° C.-150° C. The risk is that it is difficult both to achieve 90% quantum yield at room temperature and to have high thermal stability at 100° C.-150° C. The thermal stability of a phosphor's luminescence is an intrinsic property of the phosphor which is determined by the composition and the structure of the crystalline material.
Nitride and oxynitride phosphors have been used in pcLEDs because of their excellent luminescence performance at the high temperature range mentioned above. Prominent examples include metal silicon nitride based phosphors and the sialon-based phosphors. The host crystals of these phosphor materials consist mainly of chemical bonds of Si—N, Al—N, Si—O and Al—O, as well as hybrid bonds thereof, as the backbone of the structure. While these bonds are stable, the chemical bond between silicon and carbon (Si—C) has a thermal and chemical stability superior to them. Furthermore, carbon forms very stable chemical bond with many metal atoms.
The introduction of carbon or carbide into crystalline phosphor materials, however, has previously been considered detrimental in luminescence performance. The often dark body color of various metal carbides may be a source of absorption or quenching of emission light. Also, residual unreacted carbon or carbide that remains in a phosphor preparation utilizing carbon or carbide as a precursor can hinder the emission intensity of the phosphor.
Carbidonitride phosphors can be comprised of carbon, silicon, germanium, nitrogen, aluminum, boron and other metals in the host crystal and one or more metal dopants as a luminescent activator. This class of phosphors has recently emerged as a color converter capable of converting near UV (nUV) or blue light to other light in the visible spectral range, e.g., blue, green, yellow, orange and red light. The host crystal of carbidonitride phosphors is comprised of —N—Si—C—, —N—Si—N—, and —C—Si—C— networks in which the strong covalent bonds of Si—C and Si—N serve as the main building blocks of the structure. Generically, the network structure formed by Si—C bonds has a strong absorption in the entire visible light spectral region, and therefore has been previously considered unsuitable for use in host materials for high efficiency phosphors.
It has now been discovered in this invention that in certain carbidonitride phosphor compositions, carbon actually enhances, rather than quenches, the luminescence of a phosphor, in particular at relatively high temperatures (e.g. 200° C.-400° C.). The invention demonstrates that the reflectance of certain silicon carbidonitride phosphors in the wavelength range of the desired emission spectrum increases as the amount of carbon increases within certain preferred ranges. These carbidonitride phosphors have excellent thermal stability of emission and high emission efficiency.